In a UMTS (Universal Mobile Telecommunications System) network, for the purposes of achieving higher-speed data rates, providing low delay and so on, long-term evolution (LTE) has been standardized (see Non Patent Literature 1). For the purposes of achieving further broadbandization and higher speed beyond LTE (also called LTE Rel. 8 or 9), LTE-A (also called LTE advanced, LTE Rel. 10, 11 or 12) has been standardized and successor systems to LTE have been also studied a (for example, such a system is known as, FRA (Future Radio Access), 5G (5th Generation Mobile Communication System), LTE Rel. 13, Rel. 14, etc.)
In LTE Rel. 10/11, in order to achieve broadbandization, there has been introduced carrier aggregation (CA) in which a plurality of component carriers (CCs) are aggregated. Each CC corresponds one unit of the system band of LTE Rel. 8. In CA, a user terminal (UE: User Equipment) is configured with a plurality of CCs of the same radio base station (eNB: eNodeB).
On the other hand, there has been also introduced dual connectivity (DC) in which UE is configured with a plurality of cell groups (CGs) of different radio base stations. Each cell group consists of at least one cell (CC). Since a plurality of CCs of different radio base stations are aggregated in DC, DC is also called inter-eNB CA.
In addition, in LTE Rel. 8-12, there have been introduced frequency division duplex (FDD) in which downlink (DL) transmission and uplink (UL) transmission are performed using different frequency bands and time division duplex (TDD) in which DL transmission and UL transmission are performed using the same frequency band and in a time-switching manner.